1. Field of the Invention
This invention relates generally to a system and method for efficiently providing anode recirculation to a fuel cell stack and, more particularly, to a system and method for efficiently providing anode recirculation gas to the anode side of the fuel cell stack using an injector/ejector.
2. Discussion of the Related Art
Hydrogen is a very attractive fuel because it is clean and can be used to efficiently produce electricity in a fuel cell. A hydrogen fuel cell is an electro-chemical device that includes an anode and a cathode with an electrolyte therebetween. The anode receives hydrogen gas and the cathode receives oxygen or air. The hydrogen gas is dissociated in the anode to generate free protons and electrons. The protons pass through the electrolyte to the cathode. The protons react with the oxygen and the electrons in the cathode to generate water. The electrons from the anode cannot pass through the electrolyte, and thus are directed through a load to perform work before being sent to the cathode.
Proton exchange membrane fuel cells (PEMFC) are a popular fuel cell for vehicles. The PEMFC generally includes a solid polymer electrolyte proton conducting membrane, such as a perfluorosulfonic acid membrane. The anode and cathode typically include finely divided catalytic particles, usually platinum (Pt), supported on carbon particles and mixed with an ionomer. The catalytic mixture is deposited on opposing sides of the membrane. The combination of the anode catalytic mixture, the cathode catalytic mixture and the membrane define a membrane electrode assembly (MEA). MEAs are relatively expensive to manufacture and require certain conditions for effective operation.
Several fuel cells are typically combined in a fuel cell stack to generate the desired power. For example, a typical fuel cell stack for a vehicle may have two hundred or more stacked fuel cells. The fuel cell stack receives a cathode input reactant gas, typically a flow of air forced through the stack by a compressor. Not all of the oxygen is consumed by the stack and some of the air is output as a cathode exhaust gas that may include water as a stack by-product. The fuel cell stack also receives an anode hydrogen reactant gas that flows into the anode side of the stack. The stack also includes flow channels through which a cooling fluid flows.
The fuel cell stack includes a series of bipolar plates positioned between the several MEAs in the stack, where the bipolar plates and the MEAs are positioned between two end plates. The bipolar plates include an anode side and a cathode side for adjacent fuel cells in the stack. Anode gas flow channels are provided on the anode side of the bipolar plates that allow the anode reactant gas to flow to the respective MEA. Cathode gas flow channels are provided on the cathode side of the bipolar plates that allow the cathode reactant gas to flow to the respective MEA. One end plate includes anode gas flow channels, and the other end plate includes cathode gas flow channels. The bipolar plates and end plates are made of a conductive material, such as stainless steel or a conductive composite. The end plates conduct the electricity generated by the fuel cells out of the stack. The bipolar plates also include flow channels through which a cooling fluid flows.
Some fuel cell systems employ anode recirculation where the anode exhaust gas is sent back to the anode input so that the unused hydrogen in the exhaust can be reused. Typically a pump is required in the anode recirculation loop so that the proper pressure and ratio of recirculation gas to fresh hydrogen is provided to the anode input of the stack to provide efficient stack operation. An improper anode flow and pressure to the fuel cell stack could result in a low anode flow rate that may allow water to accumulate in the anode flow channels. Accumulation of water in the anode flow channels may cause anode flow to be diverted to other channels where those channels feeding certain fuel cells in the stack are starved of hydrogen, and may experience a voltage collapse. In one embodiment, it is necessary to provide about half as much recirculation gas as fresh hydrogen being provided to the anode side of the stack. If enough recirculation gas is not provided, then the flow rate may not be high enough to drive out excess water in the anode flow channels. The lower the current density of the stack, the lower the anode flow rate, and thus the more likely the stack will be flooded with water.
An improper anode flow and pressure to the fuel cell stack could also result in a low anode flow rate that may not provide hydrogen gas to each fuel cell in the stack. As mentioned, a typical fuel cell stack may have two hundred or more fuel cells. Tolerances in material properties might cause a higher pressure drop for some of the fuel cells, thus reducing the amount of gas supplied to an individual fuel cell. If enough recirculation gas is not provided, then the flow rate may not be high enough to supply sufficient hydrogen gas to the fuel cells with a higher individual pressure drop.
U.S. Patent Application Publication No. 2006/024548 to Pechtold et al. discloses an injector/ejector for a fuel cell system that injects a combination of anode fuel and anode exhaust into the anode side of a fuel cell stack. The injector/ejector is designed to eliminate the need for a recirculation pump in an anode recirculation system. However, improvements can be made for using an injector/ejector for providing the proper ratio of fresh hydrogen and anode recirculation gas to the anode side of the fuel cell stack for efficient stack operation.
An injector is a pulse device that when the injector is open, the flow of gas is substantially constant, and when the injector is closed, no flow is provided. Typical control of an injector uses a variable duty cycle and a fixed frequency. The duty cycle is the proportion of time the injector is open during one frequency cycle. A typical frequency for injector control in a fuel cell system may be as high as 60 Hz or as low as 15 Hz. A higher frequency is sometimes preferred for a more stable stack pressure control.
Controlling the injector with a fixed frequency causes the injector to be open for only a very short period of time when the fuel cell system is operating at low power. For example, the duty cycle can be as low as 1% at system idle. In this case, the injector is open for less than 1 ms when the injector is operated at a frequency of 15 Hz. Although this is long enough to supply the proper amount of hydrogen for the fuel cell reaction, it may be too short to build up a pressure drop across the anode side of the fuel cell stack to achieve a high enough gas velocity throughout the fuel cell stack. A certain pressure drop is required for uniform flow distribution and a high gas velocity will improve water management.